This invention relates to semiconductor diode lasers, and more particularly to structures which make it possible to enhace a lateral supermode with a predominately single-lobed farfield pattern, and discriminate efficiently against all other supermodes, and thereby achieve high power, single-lobed farfield operation. In most arrays, this desired supermode with a predominately single-lobed farfield pattern is the "fundamental" supermode.
Conventional semiconductor lasers are capable of emitting several tens of milliwatts of optical power into a single beam ten to twenty degrees wide in the junction plane (the laser arrays described herein have been shown capable of emitting 450 milliwatts of optical power into a single beam only 31/2.degree. wide). Considerable effort has gone into finding methods of increasing the power output and decreasing the beamwidth of a semiconductor laser. In principle, one method by which this might be done is to increase the width of the laser in the lateral direction to make a "broad area" laser. However, broad area lasers have very wide, poorly characterized, and unstable farfield patterns. These undesirable farfield patterns result from two physical effects.
First, the presence of a nonlinear interaction between the carriers and the optical field produces filaments, so-called because a photomicrograph of an operating device exhibits small areas of enhanced optical intensity with a filamentary structure. This interaction effectively forms a small waveguide 3 to 12 .mu.m wide within the larger waveguide defined by the entire broad area laser. These filaments are unstable, and the complicated motions and interactions of the many filaments in a broad area laser are one cause of the poor beam quality characteristic of these devices. Therefore, if a laser's power output is to be increased by increasing the laser's width, some method of stabilizing the filaments must be found. In conventional semiconductor lasers, this is usually achieved by making the laser's width narrow enough so that only one filament can form.
A second problem which must be overcome comes about because the waveguide in a wide laser can support many optical modes. In most lasers, only the fundamental mode will have a predominately single-lobed farfield pattern. Thus, if a laser's farfield pattern is to be single-lobed and diffraction limited (i.e., as narrow as possible), the fundamental mode must be the only lasing mode. All other modes must be discriminated against. This is usually achieved by making the laser narrow enough so that the waveguide supports only the fundamental mode, making it the sole lasing mode.
Thus, the twin problems of filamentation and lateral mode control may be solved by the simple expedient of limiting the width of the laser, typically 5 to 10 .mu.m. However, limiting the width of the laser strip also limits the laser's maximum power output and minimum beamwidth. Therefore, new semiconductor laser designs which achieve high power operation by increasing the laser's width must solve both the filamentation and lateral mode control problems.
One promising method of achieving high power operation is to place many semiconductor lasers in close proximity so that their optical fields add coherently. In such phase-locked semiconductor laser arrays the filamentation problem has been solved by confining the filaments within the individual laser channels which comprise the array. However, although the filamentation problem has been solved in an array, the lateral mode problem remains.
As mentioned earlier, a wide laser waveguide will support many optical modes, only one of which (the fundamental) has the desirable property of having a predominately single-lobed farfield pattern. This is also true in an array. If each of the N elemental laser waveguides within the array supports just one lateral optical mode (which is the usual case), coupled mode theory predicts that the entire array will support N "supermodes" (E. Kapon, J. Katz, and A. Yariv, "Supermode analysis of phased-locked array of semiconductor lasers", Opt. Lett. 9, 125-127, April 1984). It is therefore necessary to create an array structure in which the fundamental supermode is the preferred lasing supermode. This will occur if the fundamental supermode has a higher modal gain than all other waveguide supermodes. The modal gain is approximately given by the overlap between the spatial intensity distribution of the supermode and the gain profile within the laser. This means that it is necessary to devise an array structure in which the light intensity of the fundamental supermode is preferentially concentrated in the high gain region of the laser, and the other undesirable supermodes are concentrated in the low gain region of the laser.
Most arrays to date have been uniform arrays in which the widths of the laser channels and the spacing (i.e., interchannel thickness) between them are constant across the array. However, these arrays have suffered from undesirable double-lobed farfield patterns. This results from the fact that the fundamental and the highest order supermode have similar intensity nearfield patterns, and because the highest order supermode in a uniform array has a higher modal gain due to the presence of lossy interchannel regions. This is illustrated in FIG. 1, in which the nearfield and farfield patterns of both the fundamental and highest order supermode of a real index guided uniform array are shown by the heavy curves. (A gain-guided uniform array is similar to the real index guided case.) Note that the envelope of the nearfield patterns of both supermodes (which is given by the light dashed curves in FIG. 1) are virtually identical. The two supermodes differ only in that the highest order supermode has nulls in the lossy interchannel regions. This results in the highest order supermode in a uniform array having the highest modal gain, making it the predominate lasing mode, which in turn leads to the commonly observed undesirable twin-lobed farfield patterns of these devices.